1. Field of the Invention
This invention relates to a circuit substrate, and has particular relevance to a circuit substrate for an imaging device for imaging radiation, where the imaging device comprises an array of image cells.
2. Description of the Related Art
There are several different types of imaging devices, including: charged coupled image sensors (also known as charged coupled devices (CCDs)); semiconductor pixel detectors, which comprise a semiconductor substrate with electrodes arranged to apply depletion voltage to each pixel position and define a charge collection volume; and Active-pixel Semiconductor Imaging Devices (ASID), which comprise an array of pixel or image cells including a semiconductor substrate having an array of pixel detectors (detector cells) and a further array of pixel circuits (cell circuits). An ASID type of device is described in International patent application PCT/EP95/02056 (publication number WO95/33332). As described in WO95/33332, detector cells are arranged to generate charge in response to incident radiation, and are associated with a cell circuit, which is arranged to accumulate charge resulting from radiation incident on the detector cell. The cell circuits are individually addressable and comprise circuitry which enables charge to be accumulated from a plurality of successive radiation hits on an associated detector cell. Optionally, charge may be read out directly or individual radiation photon hits may be counted by incorporating suitable counter circuitry in the cell circuits. At a determined time, the charge or count can be read out from the cell circuit and used to generate an image based on the analogue charge or count values read from each of the cell circuits. The term “read out circuit” may also be used herein to refer to a cell circuit.
An example of an ASID is illustrated in FIG. 1. A detector layer 1 having a large number of detector cells formed in it is mounted on a circuit layer 3, which comprises a circuit chip and is formed of a semiconductor substrate having cell circuits corresponding to the detector cells formed in detector layer 1. Circuit chip 3 and detector layer 1 form an imaging device 20, and the circuit chip 3 is coupled to a mount 4, to which external interface bond wires 10 from circuit chip 3 are coupled. The bond wires 10 take signals collected from detector layer 1 by circuit chip 3 and route them to connections on mount 4. Imaging device 20 and mount 4 together form an imaging device tile 24.
Typically, the imaging surface area of device 20 is of the order of one square millimeters to several square centimeters, and, if a large imaging area is required then a plurality of tiles needs to be placed next to each other to form a large area imaging system. Due to the space taken up by the bond wires 10 there is “dead” imaging space 11 in between adjacent imaging device tiles 24. Such regions of “dead” space 11 can result in incomplete images being generated, and/or the missing data having to be compensated for, or extrapolated from the image data that was collected.
One way of addressing the problem of “dead” spaces between adjacent imaging tiles is disclosed in International Patent Application Publication No. WO98/03011, corresponding to U.S. Ser. No. 08/899,936, incorporated herein by reference. FIG. 2 schematically illustrates the arrangement disclosed in WO 98/03011. Each imaging device tile 24 includes a support 5 for the imaging device which is thus tilted such that “dead” space 11 of an adjacent tile fits under the imaging device tile 24 thereby forming a substantially continuous imaging surface. A tiled array of imaging device tiles 24 are supported with edge to edge contact on a suitable support structure 8 to form a large area substantially continuous imaging surface. A drawback of a tilted tile configuration is that the arrangement for tilting the tiles is relatively complex and involves more parts and components than would be necessary if the imaging devices 20 could be laid flat. Additionally, the imaging surface is not in a single plane which can give rise to image aberrations and artefacts. In particular, if support structure 8 is kept flat and substantially perpendicular to the direction of incident radiation, then the radiation is incident at an angle to the imaging surface thereby inducing image aberration and artefacts, and reducing image resolution due to radiation being incident on more than one detector cell as it passes through the tilted detector layer 1.
Another configuration for a tiled array of imaging device tiles 24 is disclosed in the above-mentioned international patent application (International Publication No. WO95/33332). WO95/33332 discloses an array of tiled imaging devices in which adjacent columns (a, b) of tiles are offset in a columnar direction, such as illustrated in FIG. 3. As can be seen, “dead” space 11 of tile 24 in column (a) corresponds to an imaging surface of a tile 24 in column (b). During an imaging operation, the arrangement in FIG. 3 is stepped relative to the object to be imaged in a direction transverse, preferably substantially orthogonal, to the columnar direction of the tile array. By stepping the arrangement of FIG. 3 in a transverse direction, during an image exposure, “dead” spaces 11 may be compensated for and substantially eliminated. However, such an arrangement requires a stepper mechanism for relative movement and image processing circuitry and appropriate software for processing the resultant multi exposure image. Such imaging device tile systems are complex and run the risk of mechanical failure.
A further drawback of known configurations for imaging device tiles 24 is that the detector layer 1 is mechanically and electrically coupled to the circuit layer 3 by low-temperature bump bonds. Each detector cell is coupled to a corresponding circuit cell by means of a bump bond, and consequently there is a high density array of bump bonds (e.g. in this technology the order of bump bonds per square mm is generally in the range 4 to 40K bump bonds per square mm). The following table shows spatial bump bond densities for various devices.
TABLE 1Pixel Size - side dimensionsBump Bond Density inof square in micronsbumps per square mm.500 - Gamma Camera4100 - Panoramic & Real-time Cassette100 35 - High Resolution Silicon Sensor900 10 - tested in laboratory10,000 5 - technology road map40,000
It is extremely difficult to ensure and maintain consistent bump bond quality, particularly since the bump bonds cannot be inspected. These difficulties have a significant impact on the quality and manufacturing yield of imaging devices and imaging device tiles.
The present invention is made with the foregoing considerations in mind.